The use of composite ceramic materials in applications which require resistance to impact, compression and heat generated by friction is known in the art. These characteristics cannot in fact be ensured by pure ceramic materials because of their intrinsic brittleness.
A useful application is that in braking equipment, in particular for the preparation of disks for disc brakes.
These composite ceramic materials comprise a mixture of materials comprising bundles of filaments essentially consisting of carbon, aggregating resin, pitch and other additives and are produced in accordance with the known art in the following way: the bundles of filaments are mixed with an aggregating resin, pitch and other additives and the mixture is placed in a mould where it is shaped with the help of heat and the application of pressure to yield a shaped semi-finished product. The semi-finished product is then subjected to a first firing in a furnace at a temperature such as to cause carbonization or pyrolysis of the resin.
Through the effect of this firing the semi-finished product acquires some porosity because of the loss of volatile material at the carbonization or pyrolysis temperatures. Subsequently the baked semi-finished product is subjected to a second firing in the presence of silicon at a temperature such as to cause melting of the silicon and infiltration into the pores of the aforesaid semi-finished product.
Infiltration by silicon makes it possible to increase the cohesion of the bundles of carbon filaments while at the same time the molten silicon partly reacts with the carbon of the semi-finished product under the conditions of the second firing forming silicon carbides which have the effect of improving the cohesion characteristics of the material.
Subsequently the composition of these materials has been improved through the addition of reinforcing fibres in order to prevent any fractures due to thermal and compression stresses from propagating rapidly throughout the structure of the material, bringing about its complete disintegration (patent application EP 1 124 071). The use of disks of shaped composite material comprising reinforcing fibres for disc brakes appreciably reduces the risks to the user.
Subsequently these composite ceramic materials were further improved by coating them with a layer of silicates, carbides, silicon nitrides or pure silicon. In this way surface oxidation phenomena on the material with a consequent loss of carbon and the formation of surface cavities, which give rise to a change in the system's braking characteristics over time and therefore a reduction in the performance in the vehicle to which these are fitted (international application WO 03/056206) are avoided. Using the composite material known in the art braking systems having the following advantages are obtained:
resistance to high temperatures and compression stresses,
thanks to the presence of the reinforcing fibres in the composition of the material, the propagation of fractures throughout the structure and therefore complete disintegration is avoided,
the system's braking characteristics, including the friction coefficient, remain virtually unchanged because of the application of a surface coating which protects the material from oxidation.
Notwithstanding the excellent characteristics of this composite material, it can be used to produce a braking system having a specific friction coefficient which cannot easily be changed according to the type of vehicle and braking performance requirements. In fact, in order to be able to vary the friction coefficient it would be necessary to modify the composition of the disk and/or pad material. However such a modification of the material of the braking system could, in addition to being costly, have a major impact on the mechanical properties of the material itself, which might be subject to cracking, splitting or other serious structural defects.
It is known that different types of vehicles have different braking performance requirements depending upon the use for which they are intended. For example competition motor vehicles require performance differing from that of motor vehicles in ordinary use and therefore, as far as braking is concerned, they also require different friction coefficients.
It would therefore be very convenient to be able to select a specific friction coefficient for each type of transport means (for example competition, sporting, tourist vehicles, etc.) and for this to remain substantially constant over time.